Embodiments of the present invention relate to the deposition of crystalline germanium in the fabrication of electronic, optoelectronic, and photovoltaic circuits.
Crystalline germanium (Ge) is used in many applications of electronic devices, including semiconductor, optoelectronic and photovoltaic devices, e.g., memory devices, photo detectors, and photovoltaic cells. For example, single and multi-layer crystalline germanium films are used to make high-efficiency multijunction photovoltaic cells that have multiple thin films of semiconductors such as gallium arsenide, gallium indium phosphide, and germanium p-n junctions. Each type of semiconductor has a characteristic band gap energy which causes it to absorb light most efficiently at a certain wavelength that represents a portion of the electromagnetic spectrum. Crystalline germanium layers are also used as photodetectors in optical interconnects to convert optical signals to electrical signals. Optical interconnects are used to connect optical pathways, waveguides, and optical fibers for transmitting optical signals comprising information, code or data. For example, optical interconnects are used in integrated circuit chips to replace metal interconnects in complementary metal oxide semiconductor (CMOS) structures because of their relatively smaller size, faster speed and higher data-carrying capacity than metal interconnects. Crystalline germanium is particularly useful in the manufacture of photodetectors because conventional silicon photodetectors cannot detect the near-infrared light that is often used for optical communications. These high performance photodetectors are made by forming crystalline germanium layers on dielectric substrates such as silicon dioxide, silicon nitride or other dielectric materials.
Conventional crystalline germanium layers are grown in epitaxial growth processes in which the germanium layer grows epitaxially on a seed layer which is also crystalline, typically crystalline silicon. Epitaxial growth processes are performed at high temperatures as high as 650° C. and these films grow at the relatively low growth rates of epitaxial growth. The high temperatures used during the epitaxial growth process can also cause diffusion of atoms and deterioration of underlying layers. For example, when germanium layers are epitaxially grown over CMOS structures that contain a silicon dioxide layer or metal features, processing temperatures higher than 550° C. can adversely affect the properties of the underlying CMOS structures and metal features (such as copper features) by causing diffusion of the atoms of these materials into other adjacent features. Diffusion of metal atoms into adjacent dielectric layers, such as layers of silicon dioxide, can cause deterioration of the dielectric properties of such layers. In addition, the low growth rates of epitaxial processes make them difficult to integrate with the faster CMOS production processes.
Crystalline germanium and silicon-germanium layers have also been deposited on silicon dioxide or metallic substrates using chemical vapor deposition (CVD) processes in combination with a thin polycrystalline silicon seed layer. In these methods, a seed layer comprising polycrystalline silicon is deposited in a sufficiently low thickness to reduce the impact of the electrical properties of the subsequently deposited germanium or silicon-germanium layer. After deposition of the polycrystalline silicon seed layer, a germanium layer is then deposited at elevated temperatures by chemical vapor deposition (CVD) using a germanium-containing precursor gas. However, the seed layer-CVD deposition methods provide relatively slow yields because the germanium layers can have a large incubation time (or time-to-initiate deposition). Also, the resultant deposited film can have a heterogeneous composition, be difficult to initialize, and in some cases, the germanium layer may not deposit on certain substrates.
For various reasons that include these and other deficiencies, and despite the development of crystalline germanium layers and their deposition methods, further improvements in deposition of germanium layers are continuously being sought.